1. Field of the Invention
The present invention relates to digital phase modulators. More particularly, the invention pertains to a digital phase modulator for a fiber-optic signal transmission or measurement device of the type that includes a fixed number of electrodes of different lengths arranged in parallel and on both sides of a light guidance path in or on an optical substrate.
2. Description of the Prior Art
It is known to apply the same potentials to the electrodes of a phase modulator for drive purposes. The potentials result in a positive or negative shift in the light phase at the output of the modulator depending on the positions of the electrodes with respect to the light guidance path. (Electrodes are also referred to as “positive” and “negative” electrodes.)
Patent specification DE 197 53 427 C1 discloses a low-significance component of a binary drive signal, supplied via a digital/analog converter with a downstream driver, to a specific, separate analog electrode in an otherwise digital phase modulator. This increases the accuracy of the phase modulator, which is formed from binary-weighted flat electrodes, for a fiber-optic signal transmission or measurement device (preferably for a fiber-optic interferometer). The patent specification also teaches storing correction values, which can be associated individually with the electrodes of the phase modulator, in a memory table to correct production-dependent inaccuracies in the electrode lengths and areas, and, thus, the phase modulation values. This solution increases resolution at the cost of comparatively large technical complexity that results from the need to use a D/A converter with a driver, whose analog initial values are, of limited temperature stability. When using such a phase modulator in a fiber-optic gyroscope (FOGs) with closed control loop comprising restoration electronics that produces a digital restoration signal of relatively high resolution (e.g. a 12-bit signal) for gyroscope restoration and for other fiber-optic signal-transmission and measurement devices, it is desirable that the resolution of the digital phase modulator exceed that previously possible.
One problem in the production of digitally driven integrated-optical modulators (e.g. for FOGs implemented in a multifunctional integrated-optical chip (MIOC)) is the achievable and/or reproducible resolution of electrode lengths. Based on voltage U| and a minimum length of the least significant (LSB) electrode of about 40 μm, an overall electrode length of about 10 mm is required for a 9-bit converter that can be relatively well implemented. If one were to attempt to reduce the minimum length of the LSB electrode further, considerable inaccuracies would occur as a result of field distortion. Relative accuracy of the LSB value could not be achieved, due to production tolerances. An electrode length of 80 mm would be required for a 12-bit converter. Based on such requirements for U| and the LSB length of the shortest electrode this is neither feasible nor realistic due to the considerable increase in physical length. Overall modulator length is limited to a few centimeters due to the technological constraints mentioned above and others.